Double [2+2] Photocycloaddition: Topochemical Conversion of 4

Double [2+2] Photocycloaddition: Topochemical Conversion of 4-Methyl-7-Styrylcoumarin Dimorphs into a Strained Cyclophane Jarugu Narasimha Moorthy* and Parthasarathy Venkatakrishnan

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 4 713-718

Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India ReceiVed September 28, 2006; ReVised Manuscript ReceiVed December 4, 2006

ABSTRACT: 2-Pyranone-annulated trans-stilbene, viz., 4-methyl-7-styrylcoumarin (4), crystallizes in two photoreactive plateand needle-like crystal modifications, each of which undergoes topochemically controlled stereospecific double [2+2] photocycloaddition to afford a strained tricyclic paracyclophane 6 in an isolated yield determined essentially by the molecular packing of the precursor modification. That the double photocycloaddition occurs under a strict control of the crystal lattice is demonstrated by the fact that the intermediary mono-cycloaddition product 5 does not lead to the double dimer upon photolysis either as an amorphous powder or as a thin film or in the solution state. In the context of polymorphism, the dimorphs of 4 constitute the first examples of photoreactive crystal modifications that undergo rare double [2+2] photocycloaddition reactions to yield a highly strained and otherwise inaccessible cyclophane. Introduction Because of strict control of the crystal lattice, the reactions in the solid state proceed, more often than not, with a high degree of selectivity, which may be in terms of the regio- or stereochemical outcome or product profile/distribution.1 Among the diverse solid-state reactions reported to date, [2+2] photodimerization is one that has been most extensively studied.1a Indeed, it is the pioneering work of Schmidt and co-workers on [2+2] photodimerization of cinnamic acids2 that spurred genesis of the area of solid-state photochemistry, which has come into being today as an independent branch of chemistry in general. On the basis of their extensive investigations, Schmidt and co-workers put forth the so-called “topochemical postulates”, and these have facilitated understanding of the molecular reactivity in the solid state.2a Accordingly, reactions in crystal lattices occur with minimum atomic motions, and thus the arrangement of molecules in the crystal lattices permit prediction as to the outcome of a thermal/photochemical reactions; in other words, the crystal packing is a guide to the stereochemistry of the product. The occurrence of [2+2] photocycloaddition reaction in the solid state is determined by proximity considerations between the double bonds of the reacting partners. In general, the center-to-center distance between the double bonds of the reacting partners must be e4.2 Å.3 A variety of protocols have been reported in the literature to engineer arrangement of molecules in the solid state with intermolecular contact distances conducive for [2+2] cycloaddition reactions.4 The success with such strategies has been exploited to accomplish polymerization reactions,5 synthesize cyclophanes via double photodimerization reactions,4k,6 and access novel ladderanes via sequential cycloaddition reactions in the solid state.4l Stilbene is a prototypical example that serves to illustrate the photoreactivity of olefins in general. While a variety of stilbenes are photostable in the solid state,4c their solution-state photochemistry is dominated by trans f cis isomerization7 and subsequent 6π-conrotatory closure to dihydrophenanthrenes.8 * To whom correspondence [email protected].

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Recently, we investigated the solid-state photochemistry of 2-pyranone-annulated derivatives of stilbenes, viz., 6-/7-styrylcoumarins 1-4 (Chart 1), in the quest of modifying the solidstate reactivity of stilbenes.9 Our rationale for the choice of 1-4 was based on the expectation that the stacking interactions between planar benzopyran-2-one, i.e., coumarin, moieties would steer the molecules into close distances for bimolecular cycloadditions to occur. Indeed, a variety of coumarins are known to exhibit remarkable solid-state photoreactivity via close packing of molecules with a short translational axis (ca. 4 Å) or via centrosymmetric packing of molecules due to their bipolar nature.1a,1b,10 In principle, styrylcoumarins are diolefinic and may undergo various possible photochemical reactions due to styrenic and pyrone double bonds (Chart 1) leading to a multitude of photoproducts such as isomerization products, photocyclization products, cyclobutanes, cyclophanes, polymers with cyclobutane backbones, etc. Surprisingly, we found that the cis isomers of styrylcoumarins 1-3 exhibit solid-state cis f trans isomerization, while their trans isomers are photostable.9 On the other hand, cis-4 was found to be photostable in contrast to its trans isomer, which was found to exhibit photoreactivity other than isomerization. Our sustained efforts with the crystallization of trans-4 to unravel the bimolecular reactivity in the solid state led to the recognition of two distinct crystal modifications under different crystallization conditions. Herein, we wish to disclose that each of these modifications undergoes topochemically controlled stereospecific double [2+2] photocycloaddition to afford a strained tricyclic paracyclophane 6 (vide infra) in an isolated yield determined essentially by the molecular packing of the precursor modification. Polymorphism, the phenomenon of existence of two or more crystal modifications for a compound, is of tremendous contemporary interest11 in view of the fact that different arrangements of molecules in the crystal lattices of polymorphs often lead to differences in physical properties such as melting point, particle size, enhanced shelf life, bioavailability, dissolution rates, pharmacological activity, etc.12 The results presented herein on the photobehavior of 4-methyl7-styrylcoumarin constitute the first examples of photoreactive

10.1021/cg060652k CCC: $37.00 © 2007 American Chemical Society Published on Web 03/14/2007

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Figure 1. Left: Photographs of plate-like (P) and needle-like (N) modifications of 4-methyl-7-styrylcoumarin, 4. Right: The differential scanning calorimetry (DSC) scans for 4P, 4N, and the double dimer 6 (5 °C/min under a nitrogen gas atmosphere).

Chart 1

crystal modifications that undergo a rather uncommon double [2+2] photocycloaddition reaction to yield a strained cyclophane stereospecifically.13

chloroform and xylene mixtures yielded needles (4N, see Figure 1). The differential scanning calorimetry (DSC) thermograms (Figure 1) for the two modifications show that 4P (mp 200 °C) is more stable than 4N (mp 197 °C). The two pale-yellow modifications of 4 were gently ground into tiny crystals and subjected to irradiation in a Luzchem photoreactor (λmax = 350 nm). The samples were rotated on a turntable to ensure uniform exposure of the crystals to UV radiation. Both modifications were found to be photoreactive; whereas 4P underwent conversion to the products in ca. 80%, the second modification 4N led to more than 50% recovery of the starting material in three independent photolysis reactions. A maximum of 45% yield of a colorless and highly insoluble material was isolated from 4N after 3 days of irradiation. A product with similar characteristics was isolated from 4P in 72%

Results and Discussion The synthesis of trans-strylcoumarin (4) has been reported elsewhere.9 Crystallization of 4 by slow evaporation of its solution in dichloromethane/chloroform and petroleum ether mixtures led to crystals of plate-like morphology (4P). However, a similar slow evaporation of its solution in dichloromethane/

Figure 2. The molecular structure of bis-lactol 7. The cyclophane unit and the two cyclobutane rings are shown in green and blue, respectively. The lactol moiety is disordered in the crystal lattice, and hence the hydrogens attached to the carbons are not shown.

Double [2+2] Photocycloaddition

Crystal Growth & Design, Vol. 7, No. 4, 2007 715

Figure 3. Left: A partial molecular packing diagram of the polymorph 4P. The proximate and centrosymmetrically related pairs of molecules destined for double-photodimerization across pyrone and styryl double bonds are shown in circle. Notice that the distance between the styryl double bonds of the centrosymmetrically related pair that is translated along the a-axis is 6.62 Å. Similarly, the center-to-center distance between pyrone and styryl double bonds of molecules related by the symmetry -x, -y, 1 - z is 4.92 Å. Right: Centrosymmetrically related pair of molecules that undergo double photocycloaddition. Notice that the distances between the pyrone double bond carbons and the styrene double bond carbons are ca. 3.6-3.7 Å.

yield together with 8% of a mono-cycloaddition product 5 (eq 1), which was unambiguously characterized by IR, 1H, and 13C NMR spectroscopic data (see Supporting Information). The colorless material from the two modifications was virtually insoluble in a variety of solvents and precluded direct characterization. From IR and solid-state 13C NMR spectra, the structure of the product was inferred to be a double-cycloaddition product, namely, cyclophane 6 (eq 1). This insoluble product was characterized more thoroughly (IR, 1H, and 13C NMR spectra) by reducing it with DIBAL in THF, and thereby converting it into a mixture of highly inseparable diastereomeric bis-lactols, 7 (eq 2). One of the diastereomers of the bis-lactols 7 was crystallized from a CDCl3-DMSO mixture, and the structure was unequivocally determined by X-ray diffraction studies; the molecular structure of the bis-lactol is shown in Figure 2. Indeed, the structure of the double-cycloadditionproduct is also borne out from the molecular packing of the precursor crystal modifications (vide infra). More careful experiments were carried out with the crystals of 4P. The insoluble material, characterized via chemical conversion to the lactol as the double-cycloaddition product, isolated at different conversions exhibited uniformly the same melting point (164 °C). This rules out the possibility of the formation of a polymer, which presumably is admixed with the double dimer. The product mixtures isolated at different durations of irradiations invariably yielded a mixture of monocycloaddition product 5 and the double dimer 6. The isolation and characterization of the monocycloaddition product 5 attest to the fact that it is an intermediate en route to the doublecycloaddition product 6 and that the reaction occurs sequentially according to eq 1. The influence of crystal lattice control over the formation of double-cycloaddition is demonstrated by the fact that the monocycloaddition product, isolated and subjected independently to irradiation as a powder or as a film or in solution state, does not afford the double-cycloaddition product 6 (eq 3). Thus, the formation of cyclophane 6 from the two modifications in varying yields is truly a result of crystal lattice control.

The double-cycloaddition product 6 exhibits a very strong exothermic peak at 168 °C in DSC analysis (Figure 1). When

heated at 170 °C for 0.5 h, the sample splits up to afford quantitatively a mixture of mono-cycloaddition product 5 and the precursor trans-monomer 4 (eq 4). The splitting of the cyclobutanes in the dimer was found to occur with difficulty when the solid sample was irradiated with a high-energy radiation (254 nm). Presumably, the molecular packing in this case does not permit palpable penetration of the radiation to bring about decomposition into the precursor monomers.14

To rationalize the occurrence of double dimerization in varying yields in the two crystal modifications, we have determined the single-crystal X-ray structures for both 4P and 4N. The crystals of 4P were found to belong to the space group P21/c with Z′ ) 1, while those of 4N corresponded to the space group P21/a with Z′ ) 2.15 In Figures 3 and 4 are shown the crystal packing diagrams for 4P and 4N. The molecules in the plate-like modification of trans-styrylcoumarin 4P are arranged as pairs in a centrosymmetric fashion along the b-axis (Figure 3). Each molecule interacts further with its neighbors laterally via C-H‚‚‚O interactions16 (dC-H‚‚‚O ) 2.52 Å, DC-H‚‚‚O ) 3.41 Å, θC-H‚‚‚O ) 162.4°) to form a one-dimensional (1D) layer. The two layers thus generated by the centrosymmetrically related pairs are stabilized through C-H‚‚‚π and π-π interactions. In contrast, the molecules in needle-like modification are arranged in two distinct layers along the a-axis (Figure 4). The molecules in one layer form a 1D network sustained by two C-H‚‚‚O interactions17 (dC-H‚‚‚O ) 2.61 Å, DC-H‚‚‚O ) 3.50 Å, θC-H‚‚‚O ) 161.8°, and dC-H‚‚‚O ) 2.76 Å, DC-H‚‚‚O ) 3.66 Å, θC-H‚‚‚O ) 165.0°), and such 1D networks are stabilized by C-H‚‚‚π and π-π stacking interactions. It is the molecules in these networks that are responsible for the observed double dimerization in this modification (vide infra). In contrast, the molecules in the second layer are completely devoid of C-H‚ ‚‚O hydrogen bonds and hence a 1D network. In this layer, the

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Figure 4. Top: The molecular packing arrangement in the unit cell of the polymorph 4N. Left: Arrangement of a pair of centrosymmetrically related molecules that are destined for double cycloaddition in the reactive layer. Right: Arrangement of closely packed molecules in the unreactive layer. Observe that the distances between the carbons of the double bonds are above 4.5 Å.

molecules are arranged in a zigzag fashion and are stabilized by C-H‚‚‚π and π-π aromatic stacking interactions (Figure 4). As mentioned earlier, the crystal packing is a guide to the stereochemistry of the product. A close analysis of the crystal packing of 4P in Figure 3 shows that the centrosymmetrically related molecules (-x, 1 - y, 1 - z) are oriented in such a way that the center-to-center distance between the pyrone double bond of one molecule and the styryl double bond of the other is 3.64 Å; indeed, the other geometric attributes such as the torsion angle between the double bonds, lateral displacement, etc. described by Venkatesan and Ramamurthy1a for occurrence of photodimerization in the solid state fall in the range that is generally observed for a variety of dimerization reactions (see Supporting Information). Thus, [2+2] cycloaddition at the two olefinic double bonds (styryl and pyrone) between the centrosymmetric pairs must lead to a highly strained cyclophane 6, as is observed upon photolysis; the stereochemistry of the product in eq 1 thus follows from the relative orientation of the reacting partners in the crystal lattice. Of course, the fact that such a process does occur in a stepwise manner is evidenced

from the isolation of the mono-cycloaddition product 5. It is noteworthy that mono-photocycloaddition must result in a product with a “V-shape” in which the remaining two double bonds must lie farther apart to make the second cycloaddition difficult. The AM1-minimized structure shown in Figure 5 supports this consideration. The calculated center-to-center distance between the two double bonds to form the cyclophane is ca. 5.85 Å. In agreement with this, photolysis of the monocycloaddition product 5 separately in the solid state or in the solution state does not afford the double dimer 6. The formation of double dimer in the solid-state photolysis of 4P directly emphasizes the influence of the crystal lattice in constricting the V geometry of the intermediary mono-cycloaddition product 5 such that the center-to-center distance between the remaining double bonds is within a permissible limit for the second cycloaddition to ensue. The possibility that cycloaddition between centrosymmetrically related molecules may occur sequentially to lead to the formation of a polymer is ruled out based on unsuitable orientations and distances between the double bonds of the neighboring molecules as shown in Figure 5.

Double [2+2] Photocycloaddition

Figure 5. The AM1-minimized structure of “V-shaped” monocyclized product, 5. The average distance between the carbons of the two double bonds (blue) that may undergo [2+2] cycloaddition is 5.85 Å.

The reason for why the photolysis of the crystals of 4N does not lead to quantitative yield of the double dimer as in the case of 4P emerges readily from the analysis of its crystal packing. As mentioned earlier, there are two independent molecules in the asymmetric unit cell, and one observes two distinct layers of molecules in the crystal lattice. The centrosymmetrically related molecules in one layer are oriented, in a manner very similar to that in 4P, with distance (3.58 Å) and angular requirements (see Supporting Information) between the double bonds suitable for dimerization to occur upon photoirradiation. The nearest distance between the double bonds of centrosymmetrically related pairs in the second layer is 4.61 Å with unfavorable orientations such that the molecules in this layer are virtually unreactive (cf. Supporting Information). Thus, a maximum isolated yield of 45% for the double dimer from the photolysis of the crystals of 4N is consistent with molecular packing in its crystal lattice. All considerations discussed above for 4P should be applicable here as well for the occurrence of double dimerization. Conclusion The pyran-2-one-annulated stilbene 4 crystallizes in two modifications and exhibits remarkable photoreactivity. Both crystal modifications, viz., plates and needles, undergo a rather uncommon double photodimerization to afford a strained cyclophane in varying yields. The stereochemistry of the product as well as the yield of the cyclophane is strictly determined by the crystal packing in both modifications (topochemical control). The influence of solid state is underscored by the fact that the mono-cycloaddition product 5 does not lead to the double cycloaddition product 6 when subjected to photolysis independently either as a film or in the solution state. In the context of polymorphism, the two modifications of the styrylcoumarin 4 constitute the first examples of photoreactive crystals that undergo double photocycloaddition. Experimental Section General Aspects. The DSC analyses were performed using PerkinElmer Pyris 6 differential scanning calorimeter under a nitrogen gas atmosphere at a heating rate of 5 °C min-1.

Crystal Growth & Design, Vol. 7, No. 4, 2007 717 X-ray Crystal Structure Determinations. The crystals of platelike modification of 4 for X-ray diffraction studies were grown by slow evaporation of the solution of trans-styrylcoumarin in chloroform/ dichloromethane-petroleum ether mixtures, whereas those of needlelike modification (4N) were grown from chloroform/dichloromethanexylene mixtures. The intensity data were collected at 298 K on an Enraf Nonius CAD-4 single-crystal X-ray diffractometer equipped with a molybdenum sealed tube (λ ) 0.71073 Å) and graphite monochromator. The data were collected by 2θ-θ scan mode with a variable scan speed ranging from 2.0° to a maximum of 60.0° min-1. The structure in each case was solved by Direct Methods and refined on F2 using the SHELX9718 package. All the non-hydrogen atoms were refined anisotropically. As the hydrogens were not readily revealed from difference Fourier maps, they were included in the ideal positions with fixed isotropic U values, and they were riding with their respective non-hydrogen atoms. The difference Fourier map, after the refinement, was essentially featureless in all cases. Solid-State Photolysis. The pale yellow crystals of trans-styrylcoumarin 4 (ca. 0.1-0.2 g) were crushed gently such that the crystallinity was preserved. They were spread evenly along the interior surface of a Pyrex test tube and rotated over a turntable during irradiation in a Luzchem photochemical reactor (λ = 350 nm) for 70100 h at room temperature (ca. 25 °C). The tubes were removed from the photoreactor from time to time and shaken thoroughly to avoid inhomogeneous exposure of the sample to the radiation. After irradiation, the photolysate was extracted with CHCl3, and the reaction mixture was sonicated entirely for ca. 1 h. The colorless insoluble solid was filtered at the pump and washed several times with CHCl3. The filtrate was stripped off the solvent and subjected to silica-gel column chromatography (ethyl acetate and petroleum ether mixtures) to isolate the unreacted trans-styrylcoumarin and a colorless solid product, which was characterized by IR, 1H, and 13C NMR spectroscopic techniques as monocycloaddition product 5. The solid material 6 from the photolysis of both 4P and 4N was virtually insoluble in almost all of the solvents. Only IR and solid-state 13C spectral data could be obtained to elucidate its structure. For further evidence in support of the cyclophane structure, the insoluble dimer 6 was converted into a mixture of diastereomeric lactols 7 by reaction with DIBAL-H, and the latter were characterized by IR, 1H, and 13C spectroscopic data; see Supporting Information. Monodimer 5: Colorless solid, mp 107-109 °C; IR (KBr) cm-1 3028, 2922, 1730, 1616; 1H NMR (CDCl3, 400 MHz) δ 1.63 (s, 3H), 2.36 (d, J ) 1.2 Hz, 3H), 3.36 (d, J ) 9.5 Hz, 1H), 3.81 (d, J ) 11.2 Hz, 1H), 3.99 (t, J ) 10.5 Hz, 1H), 6.12 (d, J ) 8.3 Hz, 1H), 6.22 (d, J ) 1.2 Hz, 1H), 6.82-6.85 (m, 2H), 6.92 (ABq, JAB ) 16.3 Hz, 1H), 6.99 (s, 1H), 7.00 (ABq, JAB ) 16.3 Hz, 1H), 7.15-7.22 (m, 7H), 7.29 (t, J ) 7.0 Hz, 2H), 7.42 (d, J ) 8.0 Hz, 3H); 13C NMR (CDCl3, 100 MHz) 18.6, 29.8, 43.2, 44.9, 47.4, 58.7, 114.9, 115.1, 116.8, 119.2, 121.1, 122.0, 124.1, 124.6, 126.2, 126.6, 127.0, 127.4, 128.1, 128.7, 129.6, 130.3, 136.7, 138.5, 139.9, 141.9, 151.0, 152.0, 153.5, 160.5, 166.3. Double Dimer 6: Colorless solid, IR (KBr) cm-1 2960, 2921, 1745, 1602, 1408, 1225, 1049; 13C NMR (solid-state) δ 24.9, 39.7, 45.9, 65.9, 120.4, 125.6, 127.9, 131.5, 141.5, 169.8. Characterization of Double Dimer 6 by Conversion into a BisLactol 7. The insoluble double dimer 6 isolated from the above photolysis was reduced using DIBAL-H in THF. To a suspension of 6 (0.08 g, 0.15 mmol) in dry THF (15.0 mL) contained in a 2-necked round-bottom flask was introduced DIBAL-H in toluene (0.75 mmol) at -70 °C under a nitrogen gas atmosphere. The suspension was stirred for 1 h at this temperature, and the reaction mixture was gradually allowed to attain room temperature over a period of 6 h. The reaction mixture was stirred at room temperature for further 10 h. Subsequently, it was quenched with water, and the organic matter was extracted with ethyl acetate (3 × 25 mL). The combined organic fractions were washed with brine, dried over anhyd. Na2SO4, and filtered, and solvent was removed in vacuo. The crude product was subjected to silica-gel column chromatography to isolate 0.65 g (82%) of the bis-lactol 7 as an inseparable mixture of diastereomers. The diastereomeric mixture of bis-lactol 7 was characterized by 1H, 13C NMR, and IR spectroscopic techniques (see Supporting Information). In principle, three diastereomers are possible for the lactols 7 depending on the stereochemistry of the hydroxyl groups across the cyclophane, i.e., hydroxyls “out,out” (7), “out,in” (7′) and “in,in” (7′′). The 1H NMR analysis of the lactol 7 revealed near equal yields of two

718 Crystal Growth & Design, Vol. 7, No. 4, 2007 diastereomers when initially recorded. Upon leaving the sample for 24 h in CDCl3, one of the diastereomers was found to be enriched, suggesting that second diastereomer undergoes slow conversion to the first one (see Supporting Information). The 13C NMR analysis of the sample revealed double the number of signals in addition to the fact that aromatic carbons were still larger in number. This implies that all three diastereomers presumably exist with some carbons exhibiting equivalence; see Supporting Information.

Bis-lactol 7 (a Mixture of Diastereomers): IR (KBr) cm-1 3325, 2948, 2923, 1602, 1492; 1H NMR (CDCl3+DMSO-d6, 400 MHz) δ 1.66 (s, 3H), 1.72 (s, 3H), 2.71-2.74 (s, 2H), 3.53 (t, J ) 7.3 Hz, 1H), 3.61 (t, J ) 7.6 Hz, 1H), 4.08 (s, 1H), 4.35 (s, 1H), 5.00 (s, 1H), 5.45 (s, 1H), 5.50 (s, 2H), 6.68-6.73 (m, 2H), 6.95-7.05 (m, 2H), 7.157.25 (m, 2H), 7.30-7.50 (m, 10H); 13C NMR (CDCl3+DMSO-d6, 100 MHz) 24.9, 25.1, 31.6, 35.2, 42.5, 45.3, 52.8, 53.2, 58.2, 58.9, 92.6, 94.0, 118.87, 118.94, 119.0, 119.1, 120.1, 120.2, 120.7, 120.8, 124.1, 124.2, 124.98, 125.04, 125.4, 125.8, 126.0, 126.1, 126.4, 126.5, 127.6, 127.7, 128.2, 128.4, 137.6, 138.2, 138.5, 139.2, 144.6, 144.7, 145.3, 145.4, 150.8, 151.0, 153.4, 153.6.

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Supporting Information Available: 1H and 13C NMR spectra for 5 and 7, solid-state 13C NMR spectrum for 6, and IR data for 4, 5 and 6. This material is available free of charge from Internet at http:// pubs.acs.org. The X-ray crystal structures of 4P, 4N, and 7 have been deposited at the Cambridge Crystallographic Data Centre (CCDC 619275-619277). These data can be obtained free of charge via http:// www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB21EZ, UK; fax: (+44) 1223-336-033; or [email protected]).

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